The subject application relates generally to systems and methods for visualizing subsurface regions of samples. In particular, the subject application is directed to common path systems and methods for frequency domain and time domain optical coherence tomography using non-specular reference reflection for providing internal depth profiles and depth resolved images of samples. The subject application is also directed to a delivering device for optical radiation, preferably implemented as an optical fiber probe with a partially optically transparent non-specular reflector to be used in common path frequency domain and time domain optical coherence tomography and reflectometry devices.
As known in the art, optical coherence reflectometry/tomography (OCT) involves splitting an optical radiation into at least two portions, and directing one portion of the optical radiation toward a subject of investigation. The subject of investigation will be further referred to as a “sample”, whereas the portion of optical radiation directed toward an associated sample will be further referred to as a “sample portion” of optical radiation. The sample portion of optical radiation is directed toward an associated sample by means of a delivering device, such as, for example, an optical probe. Another portion of the optical radiation, which will be further referred to as “reference portion”, is used to produce a combination optical radiation representative of an optical radiation reflected or backscattered from an associated sample.
In a typical common path OCT device, the sample and reference portions of the optical radiation propagate via the same optical path and reference reflection is created in the distal part of the OCT delivering device, which is typically implemented as an optical fiber probe. Common path OCT is insensitive to the length of the optical probe, material dispersion, and polarization changes associated with bending of the optical fiber, which makes it very easy to manufacture and user friendly. Typically, an optical power of several microwatts represents the optimal level for the power of the reference portion in common path OCT. It is also strongly preferred that the power level of the reference portion does not change as lateral (reciprocal or rotational) scanning occurs in the optical probe. Known solutions of obtaining a stable reference reflection with appropriate power level include reflection from an angle cleaved fiber tip, or specular reflection from an internal surface of the probe output window, combined with telecentric optics. Unfortunately, a telecentric optical system for the OCT optical probe requires substantially more space than a regular optical system, which makes it impractical for implementation in optical probes of critical dimensions, such as miniature endoscopic optical probes. In addition, the telecentric optical system is more expensive and difficult to assemble and align.
As to the operation of common path OCT systems, using a reflection from a tip of the optical fiber as the reference portion, is known to work perfectly for time domain OCT, however it leads to serious problems for frequency domain OCT. Even in a miniaturized probe, the optical path from the fiber tip to the sample surface and back is much larger than the intended “scanning depth”. Therefore, direct spectral analysis of the optical radiation mix coming back from the optical probe and consisting of sample and reference portions of the optical radiation, axially separated by 20 mm or more, results in very high frequency fringes and requires excessive spectral resolution of the frequency domain OCT system and is an extreme burden for the data acquisition and signal processing system. An alternative solution is to use a secondary interferometer to reduce the optical path length shift between the sample and reference portions of optical radiation to approximately 1 mm or less.
However, this solution for common path frequency domain OCT is prone to an additional noise originating from interference between two replicas of the reference radiation, which can make questionable a practical realization of the secondary interferometer layout. In time domain common path OCT systems, a secondary interferometer is necessarily required, because the optical path difference between reference and sample potions of the optical radiation has to be scanned to obtain an in-depth profile. Fortunately, the additional noise problems are not inherent to time domain common path OCT systems. However, using an angle cleaved tip of the optical fiber with high reproducibility of the cleave angle and reflection level, is technologically challenging.
Yet another solution is to build an optical system with stable specular reflection from the internal surface of the optical probe output window using non-telecentric optics. Unfortunately, in a typical OCT probe optical system using non-telecentric optics, the beam incidence angle to the probe output window changes in the course of lateral scanning. Thus, the requirements for good coupling of the optical radiation back to the optical fiber and maintaining the necessary coupling over the lateral scanning range are contradictory to each other. Therefore, it could be very problematic or impossible to get a stable level of the reference reflection from a specular reflector located in the distal part of the optical fiber probe.